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Cellular Mobile Communication Systems Lecture 9

Cellular Mobile Communication Systems Lecture 9. Engr. Shahryar Saleem Assistant Professor Department of Telecom Engineering University of Engineering and Technology Taxila EE-6211. IS-95 System Features. Digital Voice

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Cellular Mobile Communication Systems Lecture 9

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  1. Cellular Mobile Communication SystemsLecture 9 Engr. Shahryar Saleem Assistant Professor Department of Telecom Engineering University of Engineering and Technology Taxila EE-6211 TI - 1011

  2. IS-95 System Features • Digital Voice – Qualcomm Code Excited Linear Prediction fixed rate 14.4Kbps coder – variable rate QCELP coder: 9.6, 4.8, 2.4, 1.2 Kbps • As data rate reduces, the transmitter can reduce the power to achieve the same error rates • Dual Mode (AMPS/CDMA), Dual Band (900, 1900 MHz bands) • Low power handsets • Soft Handoff possible • Digital Data services (text, fax, circuit switched data) • Advanced Telephony Features (call waiting, voice mail, etc.) • Security: CDMA signal + Cellular Authentication Voice privacy Encryption (CAVE Algorithm) • Air Interface Standard Only TI - 1011

  3. IS-95 System Features (cont) • Code Division Multiple Access/FDMA/FDD • Traffic Channel – Pair of 1.25 MHz radio channels (up/downlink) – Several users share a radio channel separated by a code not a timeslot or frequency! – Receiver performs a time correlation operation to detect only desired codeword – All other codewords appear as noise due to decorrelation – Receiver needs to know only codeword and frequency used by transmitter – Adjust power often to prevent near –far problem • Universal frequency reuse (frequency reuse cluster size K =1) – Simple planning – large capacity increase TI - 1011

  4. Universal Frequency Reuse TI - 1011

  5. CDMA • CDMA Advantages – resistant to narrow band interference – resistant to multipath fading and ISI – no hard limit on number of users (soft capacity) • As number of users on a frequency increase the BER goes up though – frequency reuse cluster size of K = 1 !!!! • Large Capacity Increase compared to TDMA, FDMA • Disadvantages – Implementation complexity of spread spectrum – Synchronization requirements – Power control is essential for practical operation • Used in cdmaone and both 3G standards TI - 1011

  6. CDMA Properties: Multipath Combining • Multipath: reflection, diffraction, and dispersion of the signal energy caused by natural obstacles such as buildings or hills, or multiple copies of signals sent intentionally (soft handover) • Rake receiver used to combine different path components: each path is despread separately by “fingers” of the Rake receiver and then combined • Possible due to “low auto- -correlation” of spreading code TI - 1011

  7. Rake Receiver TI - 1011

  8. Multipath Resolution and the Rake Receiver TI - 1011

  9. IS-95 CDMA Radio Aspects • IS-95 is an air interface standard only • System use FDD/FDMA/CDMA • FDD => Uplink and Downlink channels separated according to Cellular band or PCS band regulatory requirements • Bandwidth after spreading is 1.23 MHz with guardband becomes 1.25 MHz • IS-95a standard designed for AMPS (800 MHz) cellular band – Each cellular provider is allocated 25 MHz spectrum => ten 1.25-MHz CDMA duplex channels if A AMPS Band provider, 9 if B band provider – Channel operates at 1.228 Mchips/sec – A 64 bit spreading code is used (Walsh Code) – Modulation is QPSK and slight variations of QPSK TI - 1011

  10. Physical Channels • A CDMA system has 1.25 MHz wideband carriers – Carrier bandwidth in AMPS is 30 kHz – Carrier bandwidth in GSM is 200 kHz – Carrier bandwidth in IS-95 is 1.23 MHz – 1.25MHz with guard band • One CDMA carrier can contain 41 AMPS channels (41x30=1230) • In Cellular Band IS-95 carrier frequencies are denoted in terms of the AMPS channel numbers TI - 1011

  11. Interference between CDMA and AMPS/TDMA Systems • The recommended guard band between the CDMA carrier band edge and an AMPS or TDMA carrier is 270 KHz => 9 AMPS channels of 30 kHz • To set up one CDMA channel, 59 AMPS channels have to be cleared (1.77 MHz) • To set up two CDMA channels, only 100 AMPS channels have to be cleared (3 MHz) TI - 1011

  12. Modulationand Coding Features TI - 1011

  13. Codes Used in IS-95 Systems • Walsh codes • They are the “orthogonal codes” used to create “logical channels” on the up/downlink (at the same time and within the same frequency band) • The orthagonal codes are used to isolate the transmissions between different channels within a cell • PN (pseudo-noise) codes • They are used to distinguish between transmissions from different cells and are generated using “linear feedback shift registers” • Basically a pseudo-random number generator • They have excellent autocorrelation and cross correlation properties • Two short PN codes and a long PN code are used in IS-95 that have periods of 215 – 1 and 242 – 1 • Convolutional codes for error correction • Block codes with interleaving and error correction TI - 1011

  14. Sample IS-95 System Identifiers TI - 1011

  15. IS-95 Logical Channels • CDMA systems define multiple channels per frequency channel • Pilot channel – Provides a reference to all signals (beacon) • Sync channel – Used for obtaining timing information • Paging channel – Used to “page” the mobile terminal when there is an incoming call • Traffic channel – Carries actual voice or data traffic : fundamental code channel • Up to seven supplemental code channels TI - 1011

  16. IS-95 CDMA Channels TI - 1011

  17. Basic Spreading Procedure On The Forward Channel in IS-95 • Symbols (after coding, interleaving) are generated at different rates • The symbols are modulated by Walsh codes which are obtained from Hadamard Matrices (see text book, page 372) • Each Walsh code identifies one of the 64 forward channels • After spreading the symbols by orthogonal codes, they are further scrambled in the in-phase and quadrature phase lines by short PN-spreading codes (length 15 and period of 32,768 chips) • PN –spreading codes are not orthogonal but possess good autocorrelation and cross-correlation properties TI - 1011

  18. Basic Spreading Procedure On The Forward Channel in IS-95 TI - 1011

  19. IS-95 Forward Channel TI - 1011

  20. Pilot Channel • It is continuously transmitted by a BS on the forward link • Like a “beacon” (Compare BCCH in GSM) • Acts as the reference signal for all MSs • Used to lock onto all other logical channels • Used in demodulation and coherent detection • Used to measure RSS for handoff and open loop power control • It uses all the zero Walsh code (W0) and contains no information except the RF carrier • It is also spread using the PN sequence code to identify the BS TI - 1011

  21. Pilot Channel Creation TI - 1011

  22. Sync Channel • The sync channel is used to acquire initial time synchronization • It uses Walsh code W32 for spreading • Sync channel uses the same PN spreading code for scrambling as the pilot channel • The sync channel data operates at 1,200 bps • After a rate ½ convolution encoding, the data rate in increased to 2400 bps, repeated to 4800 bps and then block interleaving is employed • The sync message includes the system and network identification, the offset of the PN-short code, the state of the PN-long code and the paging channel data rate (4.8 or 9.2 kbps) TI - 1011

  23. Sync Channel Creation TI - 1011

  24. Paging Channel • Used to page the MS when there is an incoming call • Carry control messages for call setup • Employs Walsh Codes W1-W7, so there may be up to 7 paging channels • There is no power control for the pilot, sync and paging channels • The paging channel is scrambled using the PN long code of length 42 and has a period of 242 • See text for the paging channel processing page 373 TI - 1011

  25. Traffic Channel • Carry actual user information (digitally encoded voice or data) • The Forward traffic channel has two possible rate sets RS1 and Rs2 • RS1 supports data rates of 9.6, 4.8, 2.4 and 1.2 Kbps • RS2 supports data rates of 14.4, 7.2, 3.6 and 1.8 kbps • RS1 has mandatory support for IS-95 • RS2 can be optionally supported • Walsh codes W2 through W31 and W33 through W63 can be usede to spread the traffic channels depending on how many paging channels are supported in the cell TI - 1011

  26. IS-95 Reverse Channel • From MS to Base Station • On Reverse Channel the Walsh codes are not used to isolate different users, but in orthogonal signalling • Orthogonal codes are used for waveform encoding • There are no pilot or synch channels • There is an “access channel” where mobile terminals contend in random access fashion to set up a call/register location/page response TI - 1011

  27. Reverse CDMA Channel TI - 1011

  28. Waveform Encoding in IS-95 • Example: Consider the Hadamard matrix H8 • There are eight orthogonal Walsh Codes • We can perform mapping between inputs of three bits to one of the eight waveforms TI - 1011

  29. Waveform Encoding (cont) TI - 1011

  30. Waveform Encoding in IS-95 (cont) • Different mapping scheme employed in IS-95 • Consider the Walsh code of length 64 • There are 64 codes which are orthogonal to each other • If these codes are used as waveforms to represent a group of information bits, we can encode log264=6 bitsusing a Walsh code • Example: 6 bits (000000) can be transmitted using the Walsh code W0 • The Walsh code used in IS-95 is determined by the equation • i = c0 + 2c1 + 4c2 + 8c3 + 16c4 + 32c5 • Where c0 is the earliest bit and c5 is the latest bit • Example: 111010 (c5… c0) would translate into • i = 1+2x1+4x1+8x0+16x1+32x0 = 1+2+4+0+16+0 = 23 => W23 TI - 1011

  31. Two Types of IS-95 Reverse Channel • Access Channel in IS-95 • Is used by the MS to initiate communication with the BS & to respond to Paging Channel message • Fixed data rate (4800 bps) & 20 ms frame duration • Access Channel Message may carry • Origination of a call • Paging responses • Orders response • Data bursts • Acknowledgements to Paging Channel message • Registration • Basic Frame Structure: The access channel data has 96 bits every 20 ms for a data rate of 4.8 kbps – 88 bits carry the access channel data – 8 bits are encoder tail bits TI - 1011

  32. Access Channel in IS-95 TI - 1011

  33. Access Channel (cont) • There are up to 32 access channels per downlink paging channel – MSs are pseudorandomly distributed between the access channels • The 4.8 kbps data is encoded using a rate 1/3 convolutional encoder • Output of the convolutional encoder is 14.4 kbps • The output symbols are repeated to get a rate of 28.8 kbps • Every six bits is mapped into one Walsh code of 64 bits (chips) in the 64-ary orthogonal modulator TI - 1011

  34. Reverse Traffic Channel in IS-95Fundamental Code Channel • Vocoder • Reduces bit rate needed to represent speech. Operates in a variable mode of full, ½, ¼ &1/8 rates. Rate set 1 vocoder full-rate output is at 9.6 kbps and rate set 2 full rate output is at 14.4 kbps. • Convolutional Coding • Provides error detection/correction. • Symbol Repetition • Repetition of input symbols from the encoder. • Repetition is done to maintain a constant input to the block interleaver. • Full-rate symbols are not repeated and sent at full power • Half-rate repeated once & sent at half power and so on. For rate set 1 the output is maintained at 19.2 ksps (independent of vocoding rate) and for rate set 2 the output is 28.8 ksps. TI - 1011

  35. Reverse Traffic Channel in IS-95 (cont) • Orthogonal Modulation • Blocks of 6 input symbols are replaced by a corresponding 64-chip Walsh code. • Block Interleaving • Combat the effects of Rayleigh fading by ensuring that sequential data is not lost. • Data Burst Randomizer • Provides variable-rate transmission. Symbols which are repeated are deleted, .i.e., not transmitted. The transmitted duty cycle varies with the vocoder data rate and the transmission are randomized. TI - 1011

  36. Reverse Traffic Channel in IS-95 (cont) • Sequence Spreading • Provides spreading of the code. In the reverse link the data is spread using the user’s long code mask based on the ESN. • Quadrature Spreading • The channel is spread with the pilot PN sequence with a zero offset. Ensures that the mobile station is locked on to the right base station. • Baseband Filtering • Converts the signals to the cellular frequency range (800 MHz) or the PCS frequency(1900 MHz). TI - 1011

  37. Reverse Traffic Channel in IS-95Supplementary Code Channel • The supplementary code channel is primarily used for data traffic (full rate is assumed) – There is no need for a data randomizer • A single user can have many codes simultaneously to transmit data TI - 1011

  38. Mobility and Radio Resource Management in IS-95 • IS-95 uses Spread Sprectrum that brings a set of advantages not available to TDMA-based systems • Frequency reuse factor of 1 • Robust performance in the presence of Interference and Multipath • Ability to increase Capacity • Operation with a RAKE receiver is an important characteristics of CDMA (provides diversity in the presence of multipath fading to improve voice quality • The figures of RAKE receiver can select either a multipath signal or a signal from another base station if it is within the range of the MS • This ability of IS-95 is employed in what is known as Soft handoffs TI - 1011

  39. Soft Handoff in IS-95 • If a mobile terminal moves away from a base station and continues to increase its transmit power to maintain contact with base station – at edge of cell will need to handoff to adjacent cell (Also near far problem) • In soft handoff a mobile terminal is required to track the pilot signals from all neighbouring base stations – It will communicate with multiple base stations simultaneously for a short while before deciding on the final candidate – This is possible because of the RAKE receiver and direct sequence spread-spectrum – Not all handoffs will be soft!– hard handoff when CDMA to AMPS and inter–CDMA frequency channel handoffs – Note soft handoff reduces system capacity as mobile tying up 2 traffic channels TI - 1011

  40. CDMA System Concepts: Soft Handovers • Mobile located in the area of overlap of multiple base stations • Transmission: – Uplink: No difference – Downlink: BSC/MSC sends out a copy of the same packet to each base station • Reception: – Uplink: Each base station demodulates packet, BSC/MSC picks the “better packet” (macro-diversity combining) – Downlink: The mobile combines the signals using a Rake receiver (micro-diversity combining) • Two power control loops TI - 1011

  41. Soft Handoff Procedure • The mobile terminal maintains a list of pilot channels that it can hear and classifies them into four categories • Active set – pilots currently used by the mobile terminal (up to three pilots can be used) • Candidate set – pilots that are not in the active set, but have sufficient signal strength for demodulation • Neighbour set – pilots of base stations of neighbouring cells that are indicated by the network through the paging channel • Remaining set – all other possible pilots in the system • Several thresholds are used by the mobile terminal to move pilots from one set to another TI - 1011

  42. Soft Handoff • IS-95 specifies three basic types of soft handoff (a) Softer: handoff between two sectors of same cell (b) Soft: handoff between sectors of adjacent cells (c) Soft-softer: candidates for handoff include two sectors from the same cell and a sector from adjacent cell • Disadvantages of Soft handoff • Call uses multiple traffic channels over air (increases interference and decreases capacity • Call uses multiple trunk in portion of wired network TI - 1011

  43. Power Control in IS-95 • In CDMA, the “near-far” problem is very significant – As users transmit at the same time and frequency, a user close to the base station may drown the signal of a user far away from the base station • To overcome this problem, power control is used – Open-loop power control • Use a transmit power that is inversely proportional to the received signal strength from a base station – Closed-loop power control • A power control bit is transmitted 800 times a second on the forward link (from BS to MS) • The bit instructs the mobile station to either increase or decrease the power by 1 dB • Power control also reduces the battery power consumption making the CDMA phones somewhat smaller than their TDMA counterparts TI - 1011

  44. Open Loop Power Control • On the access channel, the MS sends a request using a weak signal if the pilot is strong • ACK might not be received because the power was low • If no ACK is received, a stronger access probe is transmitted • This is continued a few times and then the attempt is stopped after a delay • Max number of attempts is 15 to obtain a traffic channel • Disadvantages: • Assumption is made that the forward and reverse link characteristics are identical • Slow response times (30ms) • Using the total power received from all BSs in calculation the required transmit power TI - 1011

  45. Closed Loop Power Control • On the downlink traffic channel, a power control bit is transmitted every 1.25ms (800 times per second) • A Zero bit indicates the MS to increase its transmit power • A One bits indicates the MS to decrease its transmit power • Inner- loop power control (Fast power control) • Every 1.25ms, in the BS, the receiver determines the received SIR • If the SIR is above a preset target, the MS is instructed to decrease its power by 1 dB • If the SIR is not above a preset target, the MS is instructed to increase its power by 1 dB • The control command is sent several times per frame (hence fast power control) TI - 1011

  46. Closed Loop Power Control (cont) • Outer-loop power control (slow power control) • Measures packet error rate – Changes target SIR for inner loop – Directly modify transmit power based on FER – Commands sent once per frame (hence slow power control) TI - 1011

  47. THE END TI - 1011

  48. THE END TI - 1011

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